It doesn't ultimately matter which one of you is good, and which one of you is evil. Both of you have been brought up in a society that has given you beliefs and motivations, which mean that when you see the other, you will most likely try to kill each other. It doesn't matter if those kids are evil or not, you can use any excuse you want to justify killing them, because in the end, you are either here to kill them, or here to join them. This linear video game doesn't allow for many other options, unlike actual tabletop. It's also fine if you don't want to kill the goblin children, plenty of morally questionable people don't want to hurt children, and plenty of good people do hurt children.
I once ended up in quite a bad argument with someone for almost this exact situation, except the goblins were actually trying to kill us at the time, not just tormenting small animals. We ended up negotiating with the goblins after we realised that they weren't doing any raiding or stealing and had in fact tried to trade with the nearby town. There were still plenty of other things to kill besides goblins, and we got to feel good about ourselves for using our brains for once. Did I feel bad about killing some of the goblins, not really, they were also trying to kill me at the time.
I also want to note at this point, that this game is pretty hesitant at handing out alignment penalties so far. I've played a lot of other ttrpg video games that love to slap alignment changes all over you as soon as you make even one morally grey decision, but they generally have an extremely black and white view of morality.
This is why I actually don't like the alignment system, haven't for a long time, and find it overly simplistic and without proper nuance, and I'm glad that it has virtually no effect on your player characters in 5e (unless you go plane hopping, but that's a different matter), and that alignment class locking has stopped being a thing.
And I do think you should be able to kill the tiefling children, just because the game gives you the option to side with Minthara and assault the sanctuary. It seems like an oversight, or an attempt at placating the pearl-clutching brigade to have the kids be invulnerable, while you are slaughtering all the rest of the adults.